It had heretofore been known in isolated instances in the literature that epoxy monomers may be polymerized by the action of electromagnetic radiation. For example, Penezek et al. in Die Makromolekular Chemie, 97 (1966) have reported that gamma radiation will effect polymerization of cyclohexene oxide. However, this type of reaction does not generally occur with most epoxy monomers. Additionally gamma radiation is not a convenient source of radiation and not as useful as the ultraviolet and visible regions of the spectrum. Therefore, for quite some time now, polymerization of epoxy monomers has been carried out by heating to high temperatures the monomer in which a chemical compound was incorporated, until catalysts contained therein were activated. The activation of the catalyst upon heating thereby initiated polymerization of the epoxy monomers. These methods, though successful, are unsatisfactory in that careful attention must be given to staying within the temperature limitations of the system involved. In order to prevent the harmful effects of heat curing, it is often necessary to extend the curing cycle an unreasonable length of time.
More recently, it has been discovered that epoxides may be photopolymerized employing aryl diazonium salts as photosensitive precursors. Such a procedure forms the subject matter of my U.S. Pat. No. 3,708,296 issued Jan. 2, 1973.
It has now been discovered that another class of compounds, the organohalogens, are effectively photosensitive to initiate photopolymerization of epoxides and such compounds offer a viable and attractive alternative to the use of aryl diazonium catalysts.
Organohalogens have been known for some time as free-radical initiators in photopolymerization processes where polymerization is initiated via the double bonds of an unsaturated compound. Free-radical polymerizations however are known to suffer in general from the undesirable characteristic that the polymerization is subject to inhibition by molecular oxygen. Furthermore, it is often necessary to include a free-radical inhibitor, for example, p-methoxyphenol, in such mixtures to prevent premature polymerization when the mixtures are stored in the dark.
It has now been discovered that such organohalogen compounds are effective as photoinitiators for epoxide materials which are polymerized through a cationic rather than free-radical mechanism. Moreover, it has been found that such organohalogen compounds, when combined with certain aryl diazonium salts, function synergistically to initiate photopolymerization of epoxide materials.
It has been further discovered that epoxide materials, when halogenated and subjected to the application of energy, may be photopolymerized in the absence of any additional photoinitiator.
The above discovery of the effectiveness of organohalogens as initiators for the photopolymerization of cationically polymerizable epoxides is surprising in view of their known tendency to form free-radicals on exposure to radiation and the difficulties normally experienced with free-radical initiated polymerization taken together with the fact that epoxides are known to be polymerizable through a cationic mechanism. Indeed, workers in the art have reported that organohalogen compounds are ineffective for such cationic polymerizations. See for example, Cripps et al., U.S. Pat. No. 3,347,676 issued Oct. 17, 1967, wherein polychlorinated biphenyls were ineffective to initiate photopolymerization of cationically initiated monomers. Another example is Smith, U.S. Pat. No. 3,515,552 issued June 2, 1970, which discloses that a specific class of compounds, e.g., vinyl ethers characterized by undergoing more rapid polymerization than other cationically polymerizable materials, were photopolymerized with organohalogen initiators. In this procedure, however, polymerization takes place via the double bonds in the vinyl ethers. To date, there has been no procedure which recognized that epoxide compounds, polymerizable cationically through the opening of oxirane rings rather than through double bonds, could be effectively photopolymerized employing organohalogen compounds.